1. Field of the Invention
This invention relates to a method of processing a radiation image in a radiation image recording system used for medical diagnosis and apparatus therefor. This invention particularly relates to an image gradation processing method and apparatus in a radiation image recording system in which a stimulable phosphor is used for recording radiation image information as an intermediate recording medium and the image recorded therein is read out for reproduction and finally recorded on a recording medium.
2. Description of the Prior Art
The radiation image recording system as mentioned above is disclosed, for instance, in U.S. Pat. No. 3,859,527 and Japanese patent application No. 53(1978)-84741, the latter being filed in the name of the same applicant. In this system, radiation image information is recorded in a stimulable phosphor by exposing the phosphor to a radiation transmitting through an object like a human body, then the recorded information is read out by use of a photodetector by stimulating the phosphor with a stimulating ray like a laser beam or the like, and the read out information is recorded on a recording medium by modulating a recording laser beam or the like with the information read out by the photodetector.
The above-described radiation image recording system is far more advantageous that the conventional radiography using a silver halide photographic film in that the image is recorded over a very wide dynamic range of radiation exposure. Therefore, this system is particularly valuable when it is applied to the radiographic diagnosis of the human body. By using this system for such purposes, it becomes possible to obtain the information which cannot be obtained in the conventional radiography because of the limited exposure range that can be recorded and observed on the conventional X-ray film.
Further, since the X-rays do harm to the human body, it is impossible from the viewpoint of safety to expose the human body to X-rays of high dose. Therefore, it is desired that the necessary information in the radiography can be obtained by exposing the human body only once to the X-ray of comparatively low dose. On the other hand, radiographs should preferably have both a wide exposure latitude and high image quality of high contrast, high sharpness, low noise etc. for viewing and diagnosis purposes. Unfortunately, since the conventional radiography is designed so as to satisfy all the required conditions to some extent, the range of recording density or the ability to record various levels of information and the image quality are both insufficient and neither of these properties are completely satisfied.
The present invention is particularly directed to an improvement in the radiation image recording system of the above-described type in which the image gradation of a radiation image of the chest is improved.
In the conventional chest radiography, an X-ray film is used for recording the X-ray transmission image of the chest and recorded image is observed with the naked eyes for diagnosis. In the chest radiography, there are recorded lungs, a heart and a spine. The spine has the lowest density since the transmittivity thereof to the X-rays is low. The heart has the second lowest density since the transmittivity thereof to the X-rays is comparatively low. The lungs has high density since the transmittivity thereof to the X-rays is high. Further, since the lungs have complex trachea/bronchus and blood vessels, the image of the lungs is very complicated. The part outside the substantial image of the human body has the upmost density since this part of the X-ray film is exposed to X-rays directly coming from the X-ray source.
As mentioned above, the chest radiography has various information of various parts of the human body which is recorded in the density having a wide range of levels. Sometimes, the density ranges from 0 to 3.5 in terms of optical density. Further, since the various parts are not recorded in the desirable contrast respectively, it is very difficult and necessary to have a great skill to make proper diagnosis from the radiograph in which the disease must be found out from a very slight variation in density in the image.
It is generally known in the art that the image properties can be changed by processing the image by use of an electronic signal or information processing method. For instance, even in radiography, it is possible to read the image recorded on the X-ray film by an optical scanning means and process the read out signal by a signal processing means to change the various image properties such as contrast and the density level and then record a visible image on a recording film or the like based on the processed signal.
In the radiography, however, the recorded image is used for the purpose of "diagnosis" and the diagnostic efficiency and accuracy (the level of easiness for diagnosis or adaptability to diagnosis) are not simply enhanced by simply making so-called "good" image from the point of the ordinary image quality factors such as sharpness, granularity and contrast. Rather than these factors, the diagnosis efficiency and accuracy are influenced by other complex factors such as reference with the normal shade, reference with the anatomical structure and utilization of other diagnostic view or records.
On the other hand, it has been known in the art as one example of image processing to record the radiograph on a microfilm in a reduced size. For instance, as shown in Japanese Patent Laid Open No. 48(1973)-25523, it is known to use a photographic film having a modefied gradient contrast in which the contrast (gamma) is lowered in the high density area to compress the density range at the time of copying, and further conduct an unsharp masking process to compensate the lowering of sharpness caused by image size reduction and copying. This process, however, is effective only for preventing the lowering of the image quality in the image size reduction and copying steps and is not made for the purpose of enhancing the diagnostic efficiency and accuracy.